I am using Readfully in java to read a file. The below code illustrates it.
import java.io.*;
public class RandomAccessFileDemo {
public static void main(String[] args) {
try {
// create a string and a byte array
// create a new RandomAccessFile with filename test
RandomAccessFile raf = new RandomAccessFile("/home/mayank/Desktop/Image/Any.txt", "rw");
// set the file pointer at 0 position
raf.seek(0);
int Length = (int)raf.length();
// create an array equal to the length of raf
byte[] arr = new byte[Length];
// read the file
raf.readFully(arr,0,Length);
// create a new string based on arr
String s2 = new String(arr);
// print it
System.out.println("" + s2);
} catch (IOException ex) {
ex.printStackTrace();
}
}
}
The contents of Any.txt is Hello World!!
The above code prints Hello World!!
but when I change
raf.readFully(arr,0,Length);
to
raf.readFully(arr,3,Length-3);
Instead of getting the output lo World!!, I get no error.
Can any one explain me how to use readfully.
Or how to get the output lo World!!?
readFully will start reading from the current position in the file by default. To skip the first three characters, use:
raf.skipBytes(3);
before using readFully. Also there's no reason to use an offset, so use:
raf.readFully(arr,0,Length - 3);
and things will be peachy.
IMPORTANT NOTE: This assumes that the first 3 characters are only one byte a piece, which isn't necessarily the case with some character sets. But since this is likely a beginning homework assignment or tutorial, this is likely the answer you're looking for.
Per the javadoc, the off and len parameters of readFully(byte[] b, int off, int len) affect where in your byte array the raf data is placed, not how much of the raf data is read. In all cases the remainder of the file is read fully.
If b is null, a NullPointerException is thrown. If off is negative, or
len is negative, or off+len is greater than the length of the array b,
then an IndexOutOfBoundsException is thrown. If len is zero, then no
bytes are read. Otherwise, the first byte read is stored into element
b[off], the next one into b[off+1], and so on. The number of bytes
read is, at most, equal to len.
try this instead:
raf.skipBytes(3);
raf.readFully(arr,3,Length-3);
Related
If read() method of FileInputStream return one byte and char in java occupy 2 bytes, how does casting of integer return by read() to char return character. Below is the program
import java.io.File;
import java.io.FileInputStream;
public class ReadFile {
public static void main(String[] args) throws Exception {
File file = new File("J:\\Java\\Programs\\xanadu.txt");
FileInputStream stream = new FileInputStream(file);
int i, iteration = 0;
while ((i = stream.read()) != -1) {
System.out.print((char) i);
iteration++;
}
System.out.println("\nNo of Iteration :" + iteration);
}
}
Content of file is : StackOverFlow
Output is :
StackOverflow
No of Iteration :13
So file contains 13 character which means 26 bytes. How the number of iteration is 13.
If there is a link where this behaviour is explain, please share it.
The file contains 13 ascii characters (and 1 ascii character is 1 byte). When stored in memory, in Java, each character might consumes 2 bytes. However, they are all on the basic plane... and they could be stored as UTF-8. While a single Java character might take 2 bytes of memory it might also take more when to create a single character when it's part of a String containing values from the Supplementary_Multilingual_Plane.
I am supposed to write a program using command line arguments to put in 3 different files, a human DNA sequence, a mouse DNA sequence, and an unknown sequence. Without using arrays, I have to compare each character and give the percent match as well aas which one it closely matches up to. Here is what I have so far
import java.io.File;
import java.io.FileInputStream;
import java.io.DataInputStream;
import java.io.*;
public class Lucas_Tilak_Hw8_DNA
{
public static void main (String args[]) throws IOException
{
//First let's take in each file
File MouseFile = new File(args[0]);
File HumanFile = new File(args[1]);
File UnknownFile = new File(args[2]);
//This allows us to view individual characters
FileInputStream m = new FileInputStream(MouseFile);
FileInputStream h = new FileInputStream(HumanFile);
FileInputStream u = new FileInputStream(UnknownFile);
//This allows us to read each character one by one.
DataInputStream mouse = new DataInputStream(m);
DataInputStream human = new DataInputStream(h);
DataInputStream unk = new DataInputStream(u);
//We initialize our future numerators
int humRight = 0;
int mouRight = 0;
//Now we set the counting variable
int countChar = 0;
for( countChar = 0; countChar < UnknownFile.length(); countChar++);
{
//initialize
char unkChar = unk.readChar();
char mouChar = mouse.readChar();
char humChar = human.readChar();
//add to numerator if they match
if (unkChar == humChar)
{
humRight++;
}
if (unkChar == mouChar)
{
mouRight++;
}
//add to denominator
countChar++;
}
//convert to fraction
long mouPercent = (mouRight/countChar);
long humPercent = (humRight/countChar);
//print fractions
System.out.println("Mouse Compare: " + mouPercent);
System.out.println("Human Compare: " + humPercent);
if (mouPercent > humPercent)
{
System.out.println("mouse");
}
else if (mouPercent < humPercent)
{
System.out.println("human");
}
else
{
System.out.println("identity cannot be determined");
}
}
}
If I put in random code {G, T, C, A} for each file I use, it doesn't seem to compare characters, so I get O = mouPercent and 0 = humPercent. Please Help!
Several errors in your code are to blame.
Remove the ; from the end of your for() statement. Basically, you are only reading a single character from each file, and your comparison is strictly limited to that first set of characters. It's unlikely they will have any overlap.
Second error: don't use the "file length". Characters are typically encoded as more than one byte, so you're going to get inconsistent results this way. Better to query the stream to see if there are more bytes available, and stopping when you run out of bytes to read. Most Streams or Readers have an available or ready method that will let you determine if there is more to be read or not.
Third error: DataInputStream is not going to do what you expect it to do. Read the docs -- you're getting strange characters because it's always pulling 2 bytes and building a character using a modified UTF-8 scheme, which only really maps to characters written by the corresponding DataOutput implementing classes. You should research and modify your code to use BufferedReader instead, which will more naturally respect other character encodings like UTF-8, etc. which is most likely the encoding of the files you are reading in.
TL;DR? Your loop is broken, file length is a bad idea for loop terminating condition, and DataInputStream is a special unicorn, so use BufferedReader instead when dealing with characters in normal files.
Try using floats instead of longs for your percentage variables.
I want to convert bytes in to String.
I have one android application and I am using flatfile for data storage.
Suppose I have lots of record in my flatfile.
Here in flat file database, my record size is fixed and its 10 characters and here I am storing lots of String records sequence.
But when I read one record from the flat file, then it is fixed number of bytes for each record. Because I wrote 10 bytes for every record.
If my string is S="abc123";
then it is stored in flat file like abc123 ASCII values for each character and rest would be 0.
Means byte array should be [97 ,98 ,99 ,49 ,50 ,51,0,0,0,0].
So when I want to get my actual string from the byte array, at that time I am using below code and it is working fine.
But when I give my inputString = "1234567890" then it creates problem.
public class MainActivity extends Activity {
public static short messageNumb = 0;
public static short appID = 16;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// record with size 10 and its in bytes.
byte[] recordBytes = new byte[10];
// fill record by 0's
Arrays.fill(recordBytes, (byte) 0);
// input string
String inputString = "abc123";
int length = 0;
int SECTOR_LENGTH = 10;
// convert in bytes
byte[] inputBytes = inputString.getBytes();
// set how many bytes we have to write.
length = SECTOR_LENGTH < inputBytes.length ? SECTOR_LENGTH
: inputBytes.length;
// copy bytes in record size.
System.arraycopy(inputBytes, 0, recordBytes, 0, length);
// Here i write this record in the file.
// Now time to read record from the file.
// Suppose i read one record from the file successfully.
// convert this read bytes to string which we wrote.
Log.d("TAG", "String is = " + getStringFromBytes(recordBytes));
}
public String getStringFromBytes(byte[] inputBytes) {
String s;
s = new String(inputBytes);
return s = s.substring(0, s.indexOf(0));
}
}
But I am getting problem when my string has complete 10 characters. At that time I have two 0's in my byte array so in this line
s = s.substring(0, s.indexOf(0));
I am getting the below exception:
java.lang.StringIndexOutOfBoundsException: length=10; regionStart=0; regionLength=-1
at java.lang.String.startEndAndLength(String.java:593)
at java.lang.String.substring(String.java:1474)
So what can I do when my string length is 10.
I have two solutions- I can check my inputBytes.length == 10 then make it not to do subString condition otherwise check contains 0 in byte array.
But i don't want to use this solution because I used this thing at lots of places in my application. So, is there any other way to achieve this thing?
Please suggest me some good solution which works in every condition. I think at last 2nd solution would be great. (check contains 0's in byte array and then apply sub string function).
public String getStringFromBytes(byte[] inputBytes) {
String s;
s = new String(inputBytes);
int zeroIndex = s.indexOf(0);
return zeroIndex < 0 ? s : s.substring(0, zeroIndex);
}
i think this line cause the error
s = s.substring(0, s.indexOf(0));
s.indexOf(0)
returns -1 , perhaps you should specifiy the ASCII code
for zero which is 48
so this will work s = s.substring(0, s.indexOf(48));
check documentation for indexOf(int)
public int indexOf (int c) Since: API Level 1 Searches in this string
for the first index of the specified character. The search for the
character starts at the beginning and moves towards the end of this
string.
Parameters c the character to find. Returns the index in this string
of the specified character, -1 if the character isn't found.
I want to get bytes from a string value( for Encryption/Decryption purposes ),
I have used getBytes() method for that purpose, but every time i call getBytes() method,
it returns a new bytes of array each time.
I want a unique bytes of array for a particular string.
How ? Also i want to store that information (string or byte) in a file, and i want to get back this information in form of bytes.
getBytes() will not return new byte[] every time but contents are same.
Please check the below sample
byte[] b1 = "abc".getBytes();
byte[] b2 = "abc".getBytes();
if(b1 == b2)
{
System.out.println("Equal Not possible");//Not this
}
if(Arrays.equals(b1, b2))
{
System.out.println("Equal possible");//Gets printed
}
As contents of Array are equal here it should not make any difference in any possible kind of encryption/description algorithm in whole Java world !
If String gave you the same byte array every time, it would violate the contract of the method. Here is why:
String a = "test";
byte[] abytes1 = a.getBytes();
abytes1[0] = 0; // we are modifying the byte array.
// There is no way to prevent this!
// some other caller later on does this:
byte[] abytes2 = a.getBytes();
If abytes2 was the same array as abytes1, it would have a 0 as its first entry and would not match the value of the string. String.getBytes() must create a new array every time in case a caller decides to modify the array.
Im trying to output an integer array to a file and have hit a snag. The code executes properly, no errors thrown, but instead of giving me a file containing the numbers 1-30 it gives me a file filled with [] [] [] [] [] I have isolated the problem to the included code segment.
try
{
BufferedWriter bw = new BufferedWriter(new FileWriter(filepath));
int test=0;
int count=0;
while(count<temps.length)
{
test=temps[count];
bw.write(test);
bw.newLine();
bw.flush();
count++;
}
}
catch(IOException e)
{
System.out.println("IOException: "+e);
}
filepath refers to the location of the output file. temps is an array containing the values 1-30. If anymore information is necessary, i will be happy to provide.
BufferedWriter.write(int) writes the character value of the int, not the int value. So outputing 65 should put the letter A to file, 66 would print B...etc. You need to write the String value not the int value to the stream.
Use BufferedWriter.write(java.lang.String) instead
bw.write(String.valueOf(test));
I suggest to use PrintStream or PrintWriter instead:
PrintStream ps = new PrintStream(filePath, true); // true for auto-flush
int test = 0;
int count = 0;
while(count < temps.length)
{
test = temps[count];
ps.println(test);
count++;
}
ps.close();
The problem you are having is that you are using the BufferedWriter.write(int) method. What is confusing you is that while the method signature indicates it's writing an int, it's actually expecting that int to represent an encoded character. In other words, writing 0 is writing NUL, and writing 65 would output 'A'.
From Writer's javadoc:
public void write(int c) throws IOException
Writes a single character. The character to be written is contained in the 16 low-order bits of the given integer value; the 16 high-order bits are ignored.
A simple way to correct your problem is to convert the number to a String before writing. There are numerous ways to achieve this, including:
int test = 42;
bw.write(test+"");
You could convert the integer array to a byte array and do something like this:
public void saveBytes(byte[] bytes) throws FileNotFoundException, IOException {
try (BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream(new File(filepath))) {
out.write(bytes);
}
}
You write the number as an Integer to the file, but you want it to be a string.
change bw.write(test); to bw.write(Integer.toString(test));